Masteress
The female form of a master.
"Thank YOU spaghetti masteress!"
masteress
When a gay man is in a long-term stable relationship with a partner, yet keeps a bit on the side, the bit on the side is termed the masteress. In principle, it is the same as a straight man having a mistress.
Mike was going out with Tom, but he kept Dimi as a masteress on the side.